1. Field of the Invention
The present invention relates to a method of manufacturing a modified wood material with excellent dimensional stability while preventing a discoloration thereof.
2. Description of the Prior Art
It is known that a raw wood material is treated with a formaldehyde polymer in the presence of sulfur dioxide at a reaction temperature within a reaction vessel to obtain a modified wood material. The formaldehyde polymer is selected from a formaldehyde derivative such as trioxane, tetraoxane and paraformaldehyde, etc. In addition, an accelerator is used for accelerating depolymerization of the formaldehyde polymer to formaldehyde monomer. The accelerator is selected from a chloride such as hydrogen chloride, zinc chloride, ferrous chloride, ferric chloride, magnesium chloride and ammonium chloride, and a sulfate, e.g., ferric sulfate, etc. Since adjacent hydroxyl groups of cellulose, hemicellulose or lignin, etc., in the raw wood material are bridged by formaldehyde monomer, hydrophilicity of the wood material is decreased by the resulting cross-linking structure. As a result, though the modified wood material has a weight gain of about 5 to 10%, an antiswelling efficiency thereof is increased to 50 to 60%. K. Minato, et al. describe about a method of manufacturing a modified wood material in Mokuzai Gakkaishi, Vol. 36, No. 10, P.860-866 (1990). That is, wood specimen and trioxane are provided in a heated desiccator, and then the air in the desiccator is exhausted to the outside. Immediately after a predetermined amount of sulfur dioxide is introduced into the desiccator, the desiccator is heated in an oven to vaporize trioxane and sulfur dioxide and react the wood specimen with a vapor of trioxane in the presence of a vapor of sulfur dioxide, so that a modified wood specimen having high antiswelling efficiency and moisture excluding efficiency is obtained. For example, the wood specimen is treated with the vapor of trioxane in the presence of the vapor of sulfur dioxide at a reaction temperature of 100.degree. C. for 24 hours. On the assumption that trioxane is completely depolymerized to formaldehyde monomer, the concentration of trioxane is 1.48.times.10.sup.-2 mol/dm.sup.3. On the other hand, the concentration of sulfur dioxide is in a range of 1.0.times.10.sup.-3 to 4.0.times.10.sup.-3 mol/dm.sup.3. However, in the method of Minato, et al., there is a problem of causing carbonization of the wood specimen by the vapor of sulfur dioxide prior to the reaction of the vapor of trioxane with the wood specimen, so that a discoloration of the modified wood material occurs. Particularly, in case of using the modified wood material for an interior furniture, a musical instrument and a bath tub, etc, the discoloration must be prevented. When the raw wood material is treated with the formaldehyde polymer in the presence of sulfur dioxide and ferric sulfate, it is known that the discoloration of the modified wood material is improved to some extent. However, it is not enough to use the modified wood material for these applications. In addition, there is another problem that the cross-linking structure is formed only toward a depth of 2 or 3 mm from the surface of the wood material. When a thick wood material is treated with the formaldehyde polymer, or when a volume ratio of the wood material relative to the reaction vessel is increased, it is hardly expected that the cross-linking structure is uniformly formed in the modified wood material. Consequently, good dimensional stability of the modified wood material is not obtained.